Brillouin scattering which occurs in an optical fiber changes in accordance with a strain applied to the optical fiber. A technology which measures the distribution of strains along the lengthwise direction of an optical fiber using such a phenomenon has been developed. That technology enables the measurement of the largeness of a strain by measuring the frequency change in a Brillouin scattering light, and enables the pinpointing of the strained portion of the optical fiber by measuring a time until the Brillouin scattering light comes back. Therefore, by causing an optical fiber to run across a construction, such as a bridge, a bridge column, a building, or a dam, and the material of the wing, fuel tank or the like of an aircraft, the distribution of strains applied to such a construction and a material can be detected. Based on the distribution of strains, the deterioration and the aged deterioration of the construction and the material can be known, resulting in disaster prevention and accident prevention.
According to a method of measuring a strain distribution amount which has been known so far, light pulses are input into an optical fiber, and a Brillouin scattering light scattered backwardly is measured in a time-resolving manner. According to such a method of measuring time regions by light pulses, however, the measurement time is long (taking several minutes to ten minutes), and the spatial resolution is limited (maximum: 1 m), so that the method is insufficient for an application where various constructions are dynamically managed. Therefore, users have been seeking a break-through technology which has a high spatial resolution and which can specify a portion where a strain is applied at a further short time.
To respond to such a sought, the inventors of the present invention propose, unlike the conventional time-resolving measurement method of light pulses, a technology of measuring the distribution of Brillouin scatterings along the lengthwise direction of an optical fiber by controlling the interference condition of continuous lights in Japanese Patent Publication No. 3667132 and Japanese Patent Publication No. 3607930, and acquire a patent for that technology. The technology is known as BOCDA (Brillouin Optical Correlation Domain Analysis), and achieves the 1 cm spatial resolution and the sampling rate of approximately 60 Hz, thereby attracting attention.
FIG. 8 is a structural diagram for an optical-fiber-characteristic measurement apparatus disclosed in Japanese Patent Publication No. 3667132. In the figure, reference numeral 101 denotes a light source which outputs a light modulated at a desired modulation frequency, and is structured in such a way that an injected current to a semiconductor laser (LD) 103 is modulated by a periodical signal generated from a signal generator 102, thereby causing the semiconductor laser 103 to generate a light undergone frequency modulation or phase modulation. The output light of the semiconductor laser 103 is divided into two by a first optical branch device 104, and the one light is input into an optical frequency modulator 105. The optical frequency modulator 105 inputs a microwave generated from a microwave generator to a light intensity modulator 108, performs amplitude modulation to generate a sideband which has a frequency difference equal to the microwave frequency with respect to the center frequency of an input light, and inputs it as a probe light into one end of a measurement-target optical fiber FUT. The other light divided by the first optical branch device 104 passes through an optical delay device 110 and a second optical branch device 111 in this order, and is input as a probe light into the other end of the measurement-target optical fiber FUT. Note that the optical delay device 110 is for setting a predetermined delay time between a pump light and a probe light. An output light from the measurement-target optical fiber FUT is divided by the second optical branch device 111, and only a lower sideband part is selected through an optical wavelength filter 112, and power thereof is measured through a light detector 113.
An explanation will now be given of the principle of Brillouin scattering. In a case where a light is input into a general optical fiber, in ultrasonic generated by thermal vibration of the glass molecules of the optical fiber material, an ultrasonic having a wavelength half of an input light wavelength is generated. Periodical change in the refraction index of glass originating from the ultrasonic work as Bragg diffraction grating, and reflect the light backwardly. This is a Brillouin scattering phenomenon. The reflected light is subjected to Doppler shift depending on the speed of the ultrasonic, and because the frequency shift amount changes in accordance with an expanding and contracting strain applied to the optical fiber, a strain can be detected by measuring the shift amount.
Specifically, two propagation waves having different frequencies, i.e., a stronger pump light and a weaker probe light are oppositely propagated into the measurement-target optical fiber FUT. At this time, as a particular phase matching condition is satisfied between the pump light and the probe light (fpump=fprobe+VB, where fpump is the center frequency of the pump light, fprobe is the center frequency of the probe light, and VB is a Brillouin frequency), acoustic phonons which scatter photons from the pump light to the probe light are generated by the interaction between both waves. This brings about the amplification of the probe light as stimulated Brillouin scattering. However, when the frequency difference between the pump light and the probe light varies largely, stimulation is suppressed.
As explained in, for example, Japanese Patent Publication No. 3667132, the basic principle of the BOCDA method is to generate intensive stimulated Brillouin scattering depending on a position along the measurement-target optical fiber FUT by performing the same frequency modulation on the pump light and the probe light propagated oppositely. Thus, according to the BOCDA method, lights from the light source 101 serve as continuous oscillation lights, the oscillation frequencies thereof are changed by the repeated waveform of a sine wave through the signal generator 102, and the optical frequency modulator 105 changes the center frequency fprobe of the probe light in such a way that the difference between the center frequency fprobe of the probe light and the center frequency fpump of the pump light becomes close to the Brillouin frequency VB. Accordingly, at most positions where the phases of the pump light and the probe light becomes asynchronous and the correlation is low, stimulation is suppressed, but at a specific narrow position (correlation position) in a cm order where the phases of the pump light and the probe light are synchronous and the correlation of both lights is high, stimulated Brillouin scattering occurs. Shifting the correlation position enables the measurement of the distribution of a strain by Brillouin scattering.
According to the apparatus shown in FIG. 8, a laser light undergone frequency modulation from the light source 101 is separated by the first optical branch device 104, the one laser light is input into the optical frequency modulator 105, modulated (intensity modulated) and input as a probe light, which has an adjustable optical frequency, into one end of the measurement-target optical fiber FUT. On the other hand, the other laser light separated by the first optical branch device 104 is subjected to a predetermined time delay by the optical delay device 110, and is input as a pump light into the other end of the measurement-target optical fiber FUT through the second optical branch device 111.
Because both of the probe light and the pump light are lights subjected to frequency modulation by the same light source 101, the probe light and the pump light input into the measurement-target optical fiber FUT indicate a periodical correlation peak along the measurement-target optical fiber FUT. At a position where the correlation peak is indicated, the optical-frequency difference between the probe light and the pump light is stable, so that the optical intensity is amplified by a stimulated Brillouin scattering phenomenon. On the other hand, at most positions other than that position, the optical frequencies of the probe light and the pump light constantly fluctuate, so that the probe light is not affected by Brillouin amplification, and the optical intensity does not change substantially. Therefore, the most part of the gain that the probe light acquires by Brillouin amplification occurs at the position where the correlation peak is indicated.
The probe light which has acquired a gain by Brillouin amplification is emitted from the other end of the measurement-target optical fiber, and then input into the optical wavelength filter 112 through the second optical branch device 111. The optical branch filter 112 selects only a low frequency sideband light from the probe light, and the intensity thereof is detected through the light detector 113.
FIG. 9 is a diagram exemplifying correlation peaks in the measurement-target optical fiber FUT in FIG. 8. In the figure, reference symbol fm denotes a frequency-modulation frequency applied to the semiconductor laser 103, and reference symbol dm denotes an interval between adjoining correlation peaks. To selectively measure stimulated Brillouin scattering occurring at a point in the measurement-target optical fiber FUT, a region where the pump light and the probe light are oppositely propagated should be limited by optical isolators to cause only one correlation peak to be present therein, as shown in the figure. Note that an explanation is given of an example case where the laser light from the semiconductor laser 103 is subjected to frequency modulation, but in a case where the laser light from the semiconductor laser 103 is subjected to phase modulation by the signal generation circuit 102, the symbol fm should be given of a different reading as a modulated frequency undergone phase modulation. That is, frequency modulation includes a phase modulation technology. As shown in FIG. 9, at a position where the probe light and the pump light input into the measurement-target optical fiber FUT indicate a correlation peak, stimulated Brillouin scattering intensively occurs. The peaks of the waveform denoted by the reference numerals 120 to 122 indicate correlation peaks, and 120 indicates a zero-order correlation peak, 121 indicates a first-order correlation, and 122 indicates a second-order correlation peak. Note that the position of the zero-order correlation peak is a position where the optical path difference between the probe light and the pump light becomes zero.
The interval dm of correlation peaks can be expressed by the following equation 1, where fm denotes the frequency-modulation frequency of the light source 101, and Vg denotes a light speed in the measurement-target optical fiber FUT.dm=Vg/2fm  [Equation 1]
According to the equation 1, it is found that the interval dm of correlation peaks is determined by the frequency-modulation frequency fm applied to the semiconductor laser 103.
FIG. 10 is a diagram showing how the positions of correlation peaks change when the frequency-modulation frequency fm is changed. As shown in the figure, as the frequency-modulation frequency fm is changed, the interval dm of correlation peaks changes, thus the positions of correlation peaks change. In this manner, the correlation peak positions, i.e., measurement positions are changed, and distribution measurement is realized. Because it is necessary to cause only one correlation peak to be present in a region sandwiched by optical isolators, the measurement range of distribution measurement becomes dm. However, the position of the zero-order correlation peak 120 does not change even if only the frequency-modulation frequency fm is changed. The symbol Δz in FIG. 10 denotes a correlation peak width which is a spatial resolution of distribution measurement.
As mentioned before, the position of the zero-order correlation peak 120 is a position where the optical path difference between the probe light and the pump light becomes zero. FIG. 11 is a diagram showing how the position of the zero-order correlation peak 120 is adjusted. As a delay time by the optical delay device 110 is adjusted, the position of the zero-order correlation peak 120 is changed as shown in FIG. 11. The position of the zero-order correlation peak 120 does not depend on the frequency-modulation frequency fm. Therefore, adjusting the delay time by the optical delay device 110 makes it possible to shift not only the zero-order correlation peak 120 but also the first-order correlation peak 121 and the second-order correlation peak 122 without changing the correlation peak interval dm. In this case, the measurement range of distribution measurement also becomes dm.
However, it makes no sense if the spatial resolutions Δz of the correlation peaks 120 to 122 become larger than the distances that the positions of the correlation peaks are shifted. Now, let us suppose that the Brillouin gain line width of the measurement-target optical fiber be ΔVB, the frequency-modulation frequency of the light source 101 be fm, the amplitude of frequency modulation of the light source 101 be Δf, and the light speed in the measurement-target optical fiber be Vg, then the spatial resolution Δz can be given by the following equation 2.Δz=(Vg·ΔVB)/2πfm·Δf  [Equation 2]
Therefore, from the equation 2, it is necessary that the spatial resolution Δz should be adjusted in such a manner as to be sufficiently small with respect to a shifted distance while adjusting, for example, the frequency-modulation frequency fm of the light source 101.
Further, the number N of available sensing points corresponding to correlation peaks in the measurement range dm of distribution measurement can be taken as evaluation parameters for the apparatus. The number N of sensing points is given by a ratio between the correlation peak interval dm and the spatial resolution Δz, and is given by the following equation.N≡dm/Δz=(π·f)/ΔVB 
According to the apparatus shown in FIG. 8, stimulated Brillouin scattering is suppressed at most positions in the measurement-target optical fiber FUT by optical frequency modulation performed by the light source 101, but at a specific correlation position, the relative frequency difference between the pump light and the probe light becomes constant, and stimulated Brillouin scattering occurs. The correlation position where stimulated Brillouin scattering occurs periodically appears along the measurement-target optical fiber FUT because frequency modulation on the probe light and the pump light are periodical. Therefore, to measure the characteristic of the measurement-target optical fiber FUT, it is necessary to insert optical isolators in such a way that only one correlation peak is present in a position in the measurement-target optical fiber FUT as mentioned before, and to adjust the delay amount of the optical delay device 110 and the frequency-modulation frequency fm applied to the semiconductor laser 103.
As shown in the foregoing equation 1, the correlation peak interval dm is inversely proportional to the frequency-modulation speed (frequency-modulation frequency fm) of the light source 101. Accordingly, when the modulation frequency fm of the light source 101 is lowered to make the frequency change thereof gradual, the correlation peak interval dm and therefore the measurement range can be made wider. However, when the correlation peak interval dm is made wide, the width of a correlation part also becomes wide, and as shown in the equation 3, the spatial resolution Δz is deteriorated and becomes a large value at the same sensing point number N. Accordingly, to make the measurement range of the apparatus wide while maintaining the spatial resolution Δz high, it is necessary to increase the amplitude (modulation amplitude) Δf the frequency modulation of the light source 101, and to increase the substantive number N of sensing points.
Regarding this point, the apparatus shown in FIG. 8 separates the increment part (signal) of the probe light from the reflection part (noise) of the pump signal that the frequency is different at approximately 10 GHz by the optical wavelength filter 112 provided ahead of the light detector 113. According to such a structure, however, it is difficult to make the amplitude Δf of the frequency modulation of the light source 101 wide unlimitedly to extend the measurement range while maintaining the spatial resolution Δz high. This is because that when the amplitude Δf exceeds approximately 5 GHz which is the half of the Brillouin frequency VB of the measurement-target optical fiber FUT, the spectra of the pump light and the probe light become overlapped, and separation of both lights by the optical wavelength filter becomes impossible. Therefore, to set the spatial resolution Δz in an order of several cm, there is no other choice to set an output light from the light source 101 to have a speed of 10 MHz or so, and the measurement range which is the length of the measurement-target optical fiber FUT is limited to 10 m or so.
An apparatus shown in FIG. 12 is proposed in Japanese Patent Publication No. 3607930 to overcome the foregoing problem. The apparatus has a pulse modulator 131 like an EO switch in lieu of the optical delay device 110 in FIG. 8, and has a timing adjuster 132 provided between the second optical branch 111 and the optical wavelength filter 112. The pulse modulator 131 modulates the other light separated by the first optical branch device 104 to a pulse-like pump light, and outputs it. The timing adjuster 132 performs gate-on in accordance with the pulse timing of the pump light, and selects one correlation peak from correlation peaks which appear repeatedly along the measurement-target optical fiber FUT.
The probe light as a continuous light is input into the one end of the measurement-target optical fiber FUT, while the pulse-like pump light is input into the other end. Accordingly, as shown in FIG. 13, in the measurement-target optical fiber FUT, the pump light (see, L2 in the figure) passes through correlation peaks P0 to PN (where N is a positive integer) in a time-oriented manner at different positions in the measurement-target optical fiber as it is propagated in the measurement-target optical fiber FUT. FIG. 13 shows how the pump light passes through correlation peaks in a time-oriented manner in the measurement-target optical fiber FUT along with the progression of the pump-light pulse.
Therefore, in considering a time when the pump light passes through the vicinity of a measurement point set in the measurement-target optical fiber FUT and a time until a light in the vicinity of the measurement point reaches the timing adjuster 132, only a light from the vicinity of the measurement point can be measured. In this way, the timing adjuster 132 has an operation timing adjusted in consideration of a timing at which the pulse modulator 131 makes a laser light pulsed, a time until the pump light emitted from the pulse modulator 131 is input into the other end of the measurement-target optical fiber FUT through the second optical branch device 111, a time until the pump light reaches the vicinity of the measurement point from the other end of the measurement-target optical fiber FUT, a time until a light from the vicinity of the measurement point reaches the other end of the measurement-target optical fiber FUT, and a time until the light reaches the timing adjuster 132 through the second optical branch device 111 from the other end of the measurement-target optical fiber FUT.
According to the apparatus shown in FIG. 12, the pulse modulator 131 cuts out pulses each having a length of 10 m or so from the laser light from the light source 101, and the timing adjuster 132 performs gating in accordance with the timings of the pulse-like pump lights, thereby selecting a correlation peak at a specific position in the measurement-target optical fiber FUT. According to this scheme, however, it is necessary to generate a narrow optical pulse having a ratio of 30 to 1 to obtain, for example, a measurement range of 300 m, the utilization efficiency of light energy decreases, and the signal accuracy is deteriorated. Moreover, the measurement time becomes long.
Regarding the BOCDA method, to detect the slight increment of a probe light due to a stimulated Brillouin phenomenon sensitively, there is known a scheme of performing intensity modulation on the pump light, and performing synchronous detection on an increased probe light from the measurement-target optical fiber FUT by a lock-in amplifier at the modulated frequency in the apparatus shown in FIG. 8. Some of the pump light, however, return to the optical detector 113 due to reflection and backward scattering in the measurement-target optical fiber FUT, and the returning light is also modulated at the synchronous detection frequency, noises are to be output from the lock-in amplifier. Therefore, it is necessary to employ a structure that the optical wavelength filter 112 is provided ahead of the optical detector 113 to select only the probe light, and when the amplitude Δf of the frequency modulation of the light source 101 is made wide to extend the measurement range while maintaining the spatial resolution Δz high, separation of both lights becomes impossible no matter what kind of optical wavelength filter 112 is used at a stage exceeding 5 GHz. As a result, it is difficult to overcome a problem such that noises increase, or the spatial resolution and the measurement range are limited.
The present invention has been made in view of the foregoing problem, and it is an object of the invention to provide a new optical-fiber-characteristic measuring apparatus and optical-fiber-characteristic measuring method which completely separate the increment of a probe light from noises, and which can extend a measurement range while maintaining a spatial resolution high.